Centrifugal compressors play a vital role in various process applications e.g.
Smooth and trouble-free operation of compressor is absolutely necessary for continuous operation of process plant. Considering this during detailed design stage, evaluation of compressor performance at various operating conditions is an important pre-order activity for Engineering Contractor. This article briefly explains generic boundary limit criteria and preferred operating envelope for Centrifugal compressor performance curve, API 617 Performance Guarantee limits and also throws lights on Impeller performance.
- Reduction of volume for storage and transport (including LNG/GTL),
- Natural gas storage in underground reservoir,
- Chemical reaction taking place at high pressure.
1. PERFORMANCE (CHARACTERISTIC CURVE)
While assessing compressor performance, following shall be taken into consideration in addition to required shaft power and polytropic efficiency.
Compressor characteristics vary with the difference in molecular weight of process gas tabulated as below. In general, the higher the molecular weight or number of stages, turndown ratio becomes less and pressure rise becomes higher.
2. PERFORMANCE GUARANTEE
2.1 According to API 617
Variable speed unit: Head and capacity shall have zero negative tolerance at the normal operating point (or other point as specified), and the power at this point shall not exceed 104% of the suppliers predicted shaft power value. This tolerance shall be inclusive of all test tolerances. Constant speed unit: The capacity shall have zero tolerance. The head shall be within the range of 100% - 105% of the normal head. The horsepower, based on measured head at normal capacity, shall not exceed 107% of the value at the specified normal operating point.
2.2 Process / Customer requirement:
Depending on the nature of the process requirement and / or customers requirement, turndown ratio and / or pressure ratio shall be specified as guaranteed requirement as agreed by Supplier.
3. IMPELLER PERFORMANCE
For evaluating impeller performance, Flow Coefficient (φ), Pressure Coefficient (ψ) and Mach number (Ma) are generally used.
Flow Coefficient (φ):
= Actual Inlet Gas Flow / Impeller Characteristic (Machine Flow)
Impeller having flow coefficient (φ) between 0.01 and 0.09 is normally considered to be within proven range. Where flow coefficient (φ) > 0.1, 3D Impellers are normally used. In such case, Suppliers experience needs to be investigated as flow passage in impeller become unusually narrow.
Head / Pressure Coefficient (ψ):
= Polytropic Head / Eulers Head
Hp = Polytropic Head (m)
g = 9.8(m/sec2)
U2 = Impeller Tip Speed (m/s)
Impeller having flow coefficient (φ) in range of 0.50 to 0.55 for one stage of multistage compressor is normally considered to be within proven range.
Machine Mach Number (Ma):
= U2 / a
U2 = Impeller Tip Speed (m/s)
a = Sonic Speed (m/s)
Machine Mach number is always preferred within 1, but in case when it is exceeding 1, then Relative Mach Number (Ma rel) at Impeller Inlet shall be further investigated. In such case, Suppliers verification is must as impeller inlet diameter is generally not available to Engineering Contractor.
Senior Mechanical Engineer Rotating Equipments
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